Epidemiological studies have shown that women and men differ in their patterns of substance abuse and dependence. Sex differences in the effects of D9-tetrahydrocannabinol (THC), the primary psychoactive substituent of marijuana, have been reported in humans and in rodent models; however, the mechanisms responsible for these differences are unclear and are the focus of the proposed research. Aims 1 and 2 will determine whether sex differences in the antinociceptive and dependence-related effects of THC, respectively, are modulated by gonadal hormones, estradiol, and progesterone in females and testosterone in males. Emphasis will be on the effects of repeated THC administration on behavior, as substance abuse, by definition, requires chronic use. Dependence-related behaviors include precipitated withdrawal (somatic and affective signs), THC discrimination (a rodent model of marijuana intoxication), and conditioned place preference/aversion (CPP/CPA), a rodent model to assess the degree to which drug effects are associated with pleasure or aversion. Sex differences in tolerance to the antinociceptive effects of THC, one of the potential therapeutic indications of cannabinoid-based medications, will also be evaluated. Aim 3 will characterize the pharmacokinetics of THC and will identify regional alterations in central CB1 and CB2 receptor densities and functioning and endocannabinoid levels that are associated with behavioral changes. Understanding these basic mechanisms underlying sex differences in antinociceptive and dependence-related effects of cannabinoids will facilitate development of sex-specific approaches to treat marijuana dependence and to use cannabinoid-based medications therapeutically, specifically in the treatment of pain and spasticity. In addition, increasing scientific knowledge of endocrinological mechanisms that affect endocannabinoid system functioning could enhance understanding and treatment of a wide variety of disorders, in which dysregulation of the endocannabinoid system has been implicated, including substance abuse, psychiatric disorders, obesity, and various neurological disorders.